In my experience, it's that only the center of the door is blocked. It's happened to me on every single playthrough I've done, not different at all for any of the characters I've used. It'll seem like the sides are also inaccessible, but I can always pass through them by continually moving in that direction for a second or two.

While on the subject of the Thorn, be careful what time of day you bet on or participate in matches at. If you have a fight start very near the closing time and you enter the fight pit, ll the doors will be locked and you'll be unable to leave the fight pit or actually have a fight. Red Lucy will also become unresponsive if you place a bet too near closing time, waiting for a fight that isn't happening to end before opening the dialogue box again T_T